Behala: A pool car carrying a schoolboy and his mother on Wednesday fell headlong into a 20ft long trench in the middle of Biren Roy Road (East) that nobody had bothered to barricade.

Mother, son and the driver emerged from the accident shaken but unscathed, although the sight of the Tata Magic with its head buried in the trench suggested worse.

Dipak Prasad, one of the witnesses to the accident, said the Tata Magic was moving slowly towards Tollygunge in the rain around 6.10am when the front wheels mounted a heap of mud at the mouth of the trench. "The vehicle suddenly veered right towards the trench and I screamed. The driver probably lost control after the wheels slipped on the mud," he recounted.

Prasad was among the first to rush to the passengers' rescue. "I entered the vehicle through a window and brought out the student first. Some other people and I helped his mother out after that," he said.

Biren Roy Road (East) connects Behala Chowrasta to Tollygunge and is a busy passage by day that thousands of commuters use to reach the Metro station there. The trench, which is more than 4ft deep, can be easily missed by someone who isn't familiar with the road.

An official of the Calcutta Municipal Corporation (CMC)'s water supply department said an inquiry would be instituted to find out why the trench had not been cordoned off.

Maniklal Chatterjee, the Trinamul councillor of Ward 121 and chairman of Borough XIV, said the trench had been dug to lay a water line between the Chowrasta and Siriti booster pumping stations. Booster pumping stations help sustain the flow of water through places where "pressure" is low.

The project had started a month before Puja but there was a long festival break in between that ended only three days ago, Chatterjee said.

An engineer in the CMC said the dug-off portion should have been barricaded or cordoned off with reflector tape to warn motorists and pedestrians of the pitfall.